If you don't mind a trip upto Murcia, IKEA have a really cheap high chair and they are also next to Babys R US, where we managed to get a travel cot for around $50 euro's even before you get a mattress/covers etc
We haven't found anything else much cheaper and all the baby stuff cots, toys etc seem more expensive in Spain.
If anyone knows somewhere cheaper let us know as we always end up needing something.